US20140321800A1 - Position sensor using fiber bragg gratings to measure axial and rotational movement - Google Patents
Position sensor using fiber bragg gratings to measure axial and rotational movement Download PDFInfo
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- US20140321800A1 US20140321800A1 US14/324,465 US201414324465A US2014321800A1 US 20140321800 A1 US20140321800 A1 US 20140321800A1 US 201414324465 A US201414324465 A US 201414324465A US 2014321800 A1 US2014321800 A1 US 2014321800A1
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- 239000000835 fiber Substances 0.000 title claims abstract description 16
- 238000000034 method Methods 0.000 claims description 13
- 238000012546 transfer Methods 0.000 claims description 11
- 230000004044 response Effects 0.000 claims description 2
- 230000008859 change Effects 0.000 description 4
- 230000000875 corresponding effect Effects 0.000 description 4
- 238000006073 displacement reaction Methods 0.000 description 4
- 238000005259 measurement Methods 0.000 description 4
- 230000000694 effects Effects 0.000 description 3
- 238000005452 bending Methods 0.000 description 2
- 230000008901 benefit Effects 0.000 description 2
- 230000002596 correlated effect Effects 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 238000004891 communication Methods 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 230000006870 function Effects 0.000 description 1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/26—Auxiliary measures taken, or devices used, in connection with the measurement of force, e.g. for preventing influence of transverse components of force, for preventing overload
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/24—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
- G01L1/242—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre
- G01L1/246—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre using integrated gratings, e.g. Bragg gratings
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35306—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement
- G01D5/35309—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer
- G01D5/35316—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer using a Bragg gratings
Definitions
- the invention relates to a fiber optic sensor.
- the invention is a sensor.
- the sensor includes a fiber operable to communicate a light wave.
- the sensor also includes at least first and second Fiber Bragg Gratings disposed along the fiber.
- the sensor also includes a structure operable to be deformed in a plane of deformation.
- the at least first and second Fiber Bragg Gratings are disposed on opposite sides of the structure in the plane of deformation.
- the sensor also includes an interrogation unit operable to receive first and second signals corresponding to first and second wavelengths from the at least first and second Fiber Bragg Gratings.
- the first signal is associated with the first Fiber Bragg Grating and the second signal is associated with the second Fiber Bragg Grating.
- the sensor also includes a processor operable to derive a difference between the wavelengths of the first and second signals and compare the difference with data correlating wavelength differences to extents of deformation of the structure to yield a current extent of deformation.
- FIG. 1 is a perspective view of a first exemplary embodiment of the invention
- FIG. 2 is a front view of a second exemplary embodiment of the invention with an upper-right portion cut-away;
- FIG. 3 is a partial cross-section taken through section lines 3 - 3 in FIG. 2 ;
- FIG. 4 is a magnified detail view of the detail circle 4 in FIG. 2 ;
- FIG. 5 is a graph correlating output of Fiber Bragg Gratings to temperature
- FIG. 6 is a graph displaying the differential output of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a first temperature
- FIG. 7 is a graph displaying the differential output of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a second temperature
- FIG. 8 is a graph displaying the quotient of outputs of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a first temperature
- FIG. 9 is a graph displaying the quotient of outputs of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a second temperature.
- FIG. 10 is a graph displaying the wavelength of outputs of a Fiber Bragg Grating relative to an extent of deformation, with curves for current temperature and for a reference or known temperature.
- the invention provides an apparatus and method to measure rotational or linear displacement and temperature using Fiber Bragg Gratings (FBG).
- FBG Fiber Bragg Gratings
- the translation or rotational displacement of a shaft, with an integral wheel and screw is converted to the bending of a resilient member.
- the proportional strain (compressive and tensile) induced by the bending can then be measured by two FBGs.
- the displacement of a resilient member is induced by a spiral shaft with an integral cam is detected.
- the FBGs are fixed to opposing sides of the resilient member such that one experiences tensile strain while the other experiences compressive strain.
- FIG. 1 shows a first exemplary embodiment of the invention.
- An actuation rod 12 is supported in an outer housing by two linear bearings.
- the rod 12 contacts a wheel 14 and linear movement of the rod 12 results in rotational movement of the wheel 14 .
- the wheel 14 is fixedly mounted on a threaded axle 16 and movement of the wheel 14 results in movement of the axle 16 as well.
- a forcing block 18 which has threads that mate with the threads of the axle 16 , moves along an axis referenced at 20 .
- the block 18 can move in a first direction referenced at 24 along the axis 20 .
- the block 18 can move in a second direction along the axis 20 opposite to the first direction.
- the block 18 is generally mounted on a rail 36 .
- the rail 36 is received in a notch 38 defined by the rail 36 . Engagement between the notch 38 and the rail 36 limits movement of the block 18 along the axis 20 but does not prevent all movement.
- Movement of the forcing block 18 in the first direction 24 imparts a load on a spring member 26 .
- Distal ends (one referenced at 28 and the other hidden) defined by a pair of arms 40 , 42 of the spring member 26 are elastically deformed in the first direction 24 relative to a base portion 30 of the spring member 26 when the block 18 moves in the first direction 24 .
- the arms 40 , 42 project from the base portion 30 .
- strain is created along a length of the spring member 26 between the base portion 30 and the distal ends, in the arms 40 , 42 .
- FBG Fiber Bragg Gratings
- the FBGs 32 , 34 are in electronic communication with an interrogation unit (referenced schematically at 44 ) through the fibers 46 , 48 .
- An electronic processor 45 can be integral with or separate from the interrogation unit 44 .
- the processor 45 can process the signals received from the FBGs 32 , 34 .
- Each fiber 46 , 48 is operable to communicate a light wave and each can extend at least partially through a sheath, such as sheath 50 . It is noted that the fiber 46 , 48 are integral with one another and also with loop portion 52 , to define a continuous wave guide. As temperature affects the wavelength of a FBG, it is difficult to differentiate between wavelength change due to physical strain and the change 20 induced by thermal strain.
- the use of two FBGs in the exemplary embodiment of the invention allows for temperature compensation in strain measurement.
- the effect of thermal strains can be cancel, leaving only the strain due to mechanical deformation.
- the resultant strain is an accurate representation of the true strain in the spring member 26 . This strain can then be scaled into the desired engineering units of measure.
- the “cancelled” portion of strain corresponds to the temperature calibration of either of the FBGs 32 or 34 .
- the temperature of either FBG 32 or 34 can be calculated by detracting the known mechanical strain. This allows the embodiment of the invention to measure both a position of one of the structural components (derived from strain) and the temperature.
- FIG. 2 is a planar view of a second embodiment of the invention with a portion cut-away.
- An actuation tube 54 can be supported by an outer tube 56 .
- the tubes 54 , 56 can be concentric.
- two bearings support an internal precision spiral transfer shaft 58 .
- a nut 60 encircles the spiral transfer shaft 58 and is fixed to the actuation tube 54 .
- the nut 60 forces the spiral transfer shaft 58 to rotate in response to linear movement of the actuation tube 54 .
- Rotation of the spiral transfer shaft 58 causes rotational movement of a cam 62 .
- the cam 62 rotates it applies a load to a spring member 64 , causing the spring member 64 to bend.
- the cam 62 displaces a tip 66 (referenced in FIG. 2 ) of the spring member 64 , strain is created along a length of the spring member 64 between the tip 66 and a base portion 68 .
- FIG. 4 is taken in a plane of deformation of the spring member 64 ; the deformation of the spring member 64 is visible in this plane.
- First and second FBGs 70 , 72 are attached to the spring member 64 .
- the first FBG 70 can be attached to a top of the spring member for sensing conditions corresponding to compressive strain as the spring member 64 is deflected away, upward (relative to the perspective of FIG. 2 ) by the cam 62 .
- the second FBG 72 can be attached to bottom of the spring member 64 to sense conditions corresponding to tensile strain as the spring member 64 is deflected upward by the cam 62 .
- the FBGs 70 , 72 can be connected to an interrogation unit.
- the operation of the second embodiment is similar to the operation of the first embodiment in that the use of two FBGs allows for temperature compensation in strain measurement. This allows the second embodiment of the invention to measure both position and temperature.
- FIG. 5 the outputs of the two FBGs for an embodiment of the invention are shown for three separate temperatures as a function of the sensor position.
- Each curve (the straight lines in the graph of FIG. 5 are designated herein as curves) represents an extent of deformation of the structure being monitored.
- the two FBGs are distinguished from one another by the designations “A” and “B.”
- the horizontal axis defines the position of the sensor as a percentage and corresponds to a range over which the spring member is expected to deform in a particular operating environment. In other words, at 50% for example, the spring member will have deformed approximately 50% of the maximum amount the spring member is expected to possibly deform.
- the position of sensor is analogous to the extent of deformation of the structure being sensed, a spring member in the exemplary embodiments.
- the graph of FIG. 5 reveals that as the temperature of the FBGs increase, the outputs of the FBGs (in wavelength) also increase. Since the common increase in wavelength due to temperature affects the output of both FBGs, taking the difference of the two outputs or taking a ratio of the two outputs allows for a cancelation of thermal effects on the measurement of position change, leaving only the change due to mechanical strain.
- FIGS. 6 and 7 correlates the difference in wavelengths between FBGs A and B with the position of sensor.
- FIGS. 6 and 7 also show the difference between exemplary FBGs A and an FBG B when the pairs of FBGs are at two different temperatures.
- the FBGs A and B are at 10° C.
- the FBGs A and B are at 38° C.
- a comparison between the two graphs shows that the differential output at different temperatures yields the same position of sensor for either temperature.
- the graphs of FIGS. 6 and 7 show that mechanical strain can be accurately determined regardless of temperature.
- FIGS. 8 and 9 are analogous to FIGS. 6 and 7 .
- FIGS. 8 and 9 alternatively show the quotient of a FBG A and a FBG B with at two different temperatures, each FBG having the same temperature. As shown in FIGS. 8 and 9 , the calculated difference between the outputs of the FBGs A and B at the different temperatures yields the same position of sensor at either temperature.
- a point 74 referenced on the graph of FIG. 5 corresponds to the FBG A at 10 ° C. and at 10 % of the position of sensor.
- a point 76 referenced on the graph of FIG. 5 corresponds to the FBG B at 10° C. and at 10% of the position of sensor.
- the coordinates of point 74 are (10%, 1558.6 nanometers) and the coordinates of point 76 are (10%, 1559.6 nanometers).
- the vertical, differential distance between points 74 and 76 is 1 nanometer. This value is confirmed by reference to FIG. 6 , in which coordinates of point 78 are (10%, 1 nanometer).
- the quotient of the wavelength values (1559.6 divided by 1558.6) is equal to 1.0006.
- This value is confirmed by reference to FIG. 8 , in which coordinates of point 80 are (10%, 1.0006).
- FIG. 6 or 8 would be consulted to derive the position of sensor.
- the data graphically shown in FIGS. 6 and 8 could be in the form of a table stored in the memory of an electronic processor.
- An electronic processor can receive the signal inputs from the FBGs, determine the differential wavelength and/or quotient, access a table of data analogous to the data in FIG. 6 or 8 , and obtain the position of sensor.
- An electronic processor in an embodiment of the invention can be component of the interrogation unit.
- the temperature of the sensor can be determined.
- the measured output of one of the FBGs at the known position can be referenced against known output for a FBG at the same position and at a known temperature.
- the dashed line in FIG. 10 represents the output of an FBG at an unknown temperature.
- the solid line in FIG. 10 represents the output of an FBG at a known temperature.
- Data associated with FBG output at one or more known temperatures can be stored as data in an electronic processor that receives and processes signals from the FBGs.
- FIG. 5 shows a plurality of curves/lines representing observed FBG output at various temperatures; an electronic processor can retain such data in memory.
- the observed output of an FBG is referenced at point 82 in FIG. 10 . It has been previously determined that the position of sensor is 50%.
- the vertical position of the point 82 relative to other, known curves can be the basis of interpolation. For example, if the point 82 were vertically equidistant between a curve associated with 0° C. and a curve associated with 20° C., the temperature of the FBG could be determined to be 10° C. if the relationship between the 0° C. curve and the 20° C. curve was known to be parallel.
- the difference in wavelength can correspond directly to the temperature difference. In FIG.
- the point 82 is approximately 0.4 nanometers vertically distance from a point 84 on 0° C. curve.
- the distance 0.4 nanometers can correspond to a 40° C. temperature difference.
- the FBG operating at point 82 would thus be operating at a temperature of 40° C.
- Embodiments of the invention can be applied to methods and apparatus related to monitoring the position and temperature of mechanical components such as, by way of example and not limitation, variable valve positions, actuator stroke length, flow control devices, inlet guide vane positions, automation feedback loops, thermal growth of structures, gate position and component deflection.
- mechanical components such as, by way of example and not limitation, variable valve positions, actuator stroke length, flow control devices, inlet guide vane positions, automation feedback loops, thermal growth of structures, gate position and component deflection.
Abstract
Description
- This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/522,930 for a POSITION SENSOR USING FIBER BRAGG GRATINGS TO MEASURE AXIAL AND ROTATIONAL MOVEMENT, filed on Aug. 12, 2011, and is a divisional application of U.S. Utility patent application Ser. No. 13/584,776 for a POSITION SENSOR USING FIBER BRAGG GRATINGS TO MEASURE AXIAL AND ROTATIONAL MOVEMENT, filed on Aug. 13, 2012, both of which are hereby incorporated by reference in their entireties.
- 1. Field
- The invention relates to a fiber optic sensor.
- 2. Description of Related Prior Art
- It is known to use sensors to detect strain in a structure.
- In summary, the invention is a sensor. The sensor includes a fiber operable to communicate a light wave. The sensor also includes at least first and second Fiber Bragg Gratings disposed along the fiber. The sensor also includes a structure operable to be deformed in a plane of deformation. The at least first and second Fiber Bragg Gratings are disposed on opposite sides of the structure in the plane of deformation. The sensor also includes an interrogation unit operable to receive first and second signals corresponding to first and second wavelengths from the at least first and second Fiber Bragg Gratings. The first signal is associated with the first Fiber Bragg Grating and the second signal is associated with the second Fiber Bragg Grating. The sensor also includes a processor operable to derive a difference between the wavelengths of the first and second signals and compare the difference with data correlating wavelength differences to extents of deformation of the structure to yield a current extent of deformation.
- Advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
-
FIG. 1 is a perspective view of a first exemplary embodiment of the invention; -
FIG. 2 is a front view of a second exemplary embodiment of the invention with an upper-right portion cut-away; -
FIG. 3 is a partial cross-section taken through section lines 3-3 inFIG. 2 ; -
FIG. 4 is a magnified detail view of thedetail circle 4 inFIG. 2 ; -
FIG. 5 is a graph correlating output of Fiber Bragg Gratings to temperature; -
FIG. 6 is a graph displaying the differential output of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a first temperature; -
FIG. 7 is a graph displaying the differential output of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a second temperature; -
FIG. 8 is a graph displaying the quotient of outputs of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a first temperature; -
FIG. 9 is a graph displaying the quotient of outputs of a plurality of Fiber Bragg Gratings relative to an extent of deformation of a structure at a second temperature; and -
FIG. 10 is a graph displaying the wavelength of outputs of a Fiber Bragg Grating relative to an extent of deformation, with curves for current temperature and for a reference or known temperature. - The invention, as demonstrated by the exemplary embodiments described below, provides an apparatus and method to measure rotational or linear displacement and temperature using Fiber Bragg Gratings (FBG). In one embodiment, the translation or rotational displacement of a shaft, with an integral wheel and screw, is converted to the bending of a resilient member. The proportional strain (compressive and tensile) induced by the bending can then be measured by two FBGs. In another embodiment, the displacement of a resilient member is induced by a spiral shaft with an integral cam is detected. In various embodiments, the FBGs are fixed to opposing sides of the resilient member such that one experiences tensile strain while the other experiences compressive strain. The design of the exemplary embodiments disclosed below enables accurate displacement measurements while also measuring, and compensating for, any temperature related effects to the sensors.
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FIG. 1 shows a first exemplary embodiment of the invention. Anactuation rod 12 is supported in an outer housing by two linear bearings. Therod 12 contacts awheel 14 and linear movement of therod 12 results in rotational movement of thewheel 14. Thewheel 14 is fixedly mounted on a threadedaxle 16 and movement of thewheel 14 results in movement of theaxle 16 as well. As the threadedaxle 16 rotates, a forcingblock 18, which has threads that mate with the threads of theaxle 16, moves along an axis referenced at 20. When theaxle 16 rotates in a first angular direction referenced at 22, theblock 18 can move in a first direction referenced at 24 along theaxis 20. When theaxle 16 rotates in a second angular direction opposite to the first angular direction, theblock 18 can move in a second direction along theaxis 20 opposite to the first direction. - It is noted that the
block 18 is generally mounted on arail 36. Therail 36 is received in anotch 38 defined by therail 36. Engagement between thenotch 38 and therail 36 limits movement of theblock 18 along theaxis 20 but does not prevent all movement. - Movement of the forcing
block 18 in thefirst direction 24 imparts a load on aspring member 26. Distal ends (one referenced at 28 and the other hidden) defined by a pair ofarms spring member 26 are elastically deformed in thefirst direction 24 relative to abase portion 30 of thespring member 26 when theblock 18 moves in thefirst direction 24. Thearms base portion 30. As the forcingblock 18 displaces the distal ends of thespring member 26 relative to thebase portion 30, strain is created along a length of thespring member 26 between thebase portion 30 and the distal ends, in thearms - Two Fiber Bragg Gratings (hereafter FBG) are attached to the
spring member 26 to sense conditions that can be electronically communicated, measured, and correlated to the strain in thespring member 26, as well as correlated to the extent of movement of theblock 18, thewheel 14, and therod 12. A first FBG 32 is attached to thefirst arm 40 of thespring member 26 to sense conditions corresponding to compressive strain. A second FBG 34 is attached to thesecond arm 42 to measure tensile strain. - The FBGs 32, 34 are in electronic communication with an interrogation unit (referenced schematically at 44) through the
fibers electronic processor 45 can be integral with or separate from theinterrogation unit 44. Theprocessor 45 can process the signals received from theFBGs fiber sheath 50. It is noted that thefiber loop portion 52, to define a continuous wave guide. As temperature affects the wavelength of a FBG, it is difficult to differentiate between wavelength change due to physical strain and thechange 20 induced by thermal strain. The use of two FBGs in the exemplary embodiment of the invention allows for temperature compensation in strain measurement. By finding the difference between the wavelength changes arising fromFBGs spring member 26. This strain can then be scaled into the desired engineering units of measure. - Deriving the differential wavelength as set forth above also reveals thermal strain. The “cancelled” portion of strain corresponds to the temperature calibration of either of the
FBGs FBG -
FIG. 2 is a planar view of a second embodiment of the invention with a portion cut-away. Anactuation tube 54 can be supported by anouter tube 56. Thetubes FIG. 3 , internal to theactuation tube 54, two bearings support an internal precisionspiral transfer shaft 58. Anut 60 encircles thespiral transfer shaft 58 and is fixed to theactuation tube 54. Thenut 60 forces thespiral transfer shaft 58 to rotate in response to linear movement of theactuation tube 54. Rotation of thespiral transfer shaft 58 causes rotational movement of acam 62. As thecam 62 rotates it applies a load to aspring member 64, causing thespring member 64 to bend. As thecam 62 displaces a tip 66 (referenced inFIG. 2 ) of thespring member 64, strain is created along a length of thespring member 64 between thetip 66 and abase portion 68. -
FIG. 4 is taken in a plane of deformation of thespring member 64; the deformation of thespring member 64 is visible in this plane. First andsecond FBGs spring member 64. Thefirst FBG 70 can be attached to a top of the spring member for sensing conditions corresponding to compressive strain as thespring member 64 is deflected away, upward (relative to the perspective ofFIG. 2 ) by thecam 62. Thesecond FBG 72 can be attached to bottom of thespring member 64 to sense conditions corresponding to tensile strain as thespring member 64 is deflected upward by thecam 62. - As with the first embodiment, the
FBGs - The method of measuring strain will now be described. In
FIG. 5 , the outputs of the two FBGs for an embodiment of the invention are shown for three separate temperatures as a function of the sensor position. Each curve (the straight lines in the graph ofFIG. 5 are designated herein as curves) represents an extent of deformation of the structure being monitored. The two FBGs are distinguished from one another by the designations “A” and “B.” The horizontal axis defines the position of the sensor as a percentage and corresponds to a range over which the spring member is expected to deform in a particular operating environment. In other words, at 50% for example, the spring member will have deformed approximately 50% of the maximum amount the spring member is expected to possibly deform. Thus, the position of sensor is analogous to the extent of deformation of the structure being sensed, a spring member in the exemplary embodiments. - The graph of
FIG. 5 reveals that as the temperature of the FBGs increase, the outputs of the FBGs (in wavelength) also increase. Since the common increase in wavelength due to temperature affects the output of both FBGs, taking the difference of the two outputs or taking a ratio of the two outputs allows for a cancelation of thermal effects on the measurement of position change, leaving only the change due to mechanical strain. -
FIGS. 6 and 7 correlates the difference in wavelengths between FBGs A and B with the position of sensor.FIGS. 6 and 7 also show the difference between exemplary FBGs A and an FBG B when the pairs of FBGs are at two different temperatures. InFIG. 6 the FBGs A and B are at 10° C. and inFIG. 7 the FBGs A and B are at 38° C. A comparison between the two graphs shows that the differential output at different temperatures yields the same position of sensor for either temperature. In other words, the graphs ofFIGS. 6 and 7 show that mechanical strain can be accurately determined regardless of temperature. -
FIGS. 8 and 9 are analogous toFIGS. 6 and 7 .FIGS. 8 and 9 alternatively show the quotient of a FBG A and a FBG B with at two different temperatures, each FBG having the same temperature. As shown inFIGS. 8 and 9 , the calculated difference between the outputs of the FBGs A and B at the different temperatures yields the same position of sensor at either temperature. - A
point 74 referenced on the graph ofFIG. 5 corresponds to the FBG A at 10° C. and at 10% of the position of sensor. Apoint 76 referenced on the graph ofFIG. 5 corresponds to the FBG B at 10° C. and at 10% of the position of sensor. The coordinates ofpoint 74 are (10%, 1558.6 nanometers) and the coordinates ofpoint 76 are (10%, 1559.6 nanometers). The vertical, differential distance betweenpoints FIG. 6 , in which coordinates ofpoint 78 are (10%, 1 nanometer). The quotient of the wavelength values (1559.6 divided by 1558.6) is equal to 1.0006. This value is confirmed by reference toFIG. 8 , in which coordinates ofpoint 80 are (10%, 1.0006). - It is noted that the value of the position of sensor would be the value being pursued. After the differential wavelength or quotient is known,
FIG. 6 or 8 would be consulted to derive the position of sensor. In an embodiment of the invention, the data graphically shown inFIGS. 6 and 8 could be in the form of a table stored in the memory of an electronic processor. An electronic processor can receive the signal inputs from the FBGs, determine the differential wavelength and/or quotient, access a table of data analogous to the data inFIG. 6 or 8, and obtain the position of sensor. An electronic processor in an embodiment of the invention can be component of the interrogation unit. - Once the position of the sensor is calculated the temperature of the sensor can be determined. The measured output of one of the FBGs at the known position can be referenced against known output for a FBG at the same position and at a known temperature. The dashed line in
FIG. 10 represents the output of an FBG at an unknown temperature. The solid line inFIG. 10 represents the output of an FBG at a known temperature. Data associated with FBG output at one or more known temperatures can be stored as data in an electronic processor that receives and processes signals from the FBGs.FIG. 5 shows a plurality of curves/lines representing observed FBG output at various temperatures; an electronic processor can retain such data in memory. - The observed output of an FBG is referenced at
point 82 inFIG. 10 . It has been previously determined that the position of sensor is 50%. Several alternative methods can be applied to derive the temperature of the FBG. In one embodiment of the invention, the vertical position of thepoint 82 relative to other, known curves can be the basis of interpolation. For example, if thepoint 82 were vertically equidistant between a curve associated with 0° C. and a curve associated with 20° C., the temperature of the FBG could be determined to be 10° C. if the relationship between the 0° C. curve and the 20° C. curve was known to be parallel. Alternatively, the difference in wavelength can correspond directly to the temperature difference. InFIG. 10 , thepoint 82 is approximately 0.4 nanometers vertically distance from apoint 84 on 0° C. curve. In an embodiment of the invention, the distance 0.4 nanometers can correspond to a 40° C. temperature difference. The FBG operating atpoint 82 would thus be operating at a temperature of 40° C. - Embodiments of the invention can be applied to methods and apparatus related to monitoring the position and temperature of mechanical components such as, by way of example and not limitation, variable valve positions, actuator stroke length, flow control devices, inlet guide vane positions, automation feedback loops, thermal growth of structures, gate position and component deflection.
- While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Further, the “invention” as that term is used in this document is what is claimed in the claims of this document. The right to claim elements and/or sub-combinations that are disclosed herein as other inventions in other patent documents is hereby unconditionally reserved.
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